Modern vehicle engines have been called upon to drive an increasing number of accessories as the sophistication of modern vehicles increases, which accessories include many convenience items demanded by the customer.
Generally the accessory drive provides at least a linear ratio between the speed (rpm) of the engine and the speed (rpm) of the accessory driven shaft. At low engine speeds, no serious problems exist; but at high engine speeds serious problems exist. Because of this relationship, it is imperative to construct the accessory with proper bearings, so that it can operate at elevated speeds without an undue limitation on its life. The strength and size of rotating parts must be such that extremely high speeds do not rupture them. In addition, the wide variation in operating speed of an accessory at times creates a demand for sophisticated control systems. Such construction makes the accessory more expensive than necessary or desirable both as original equipment (OEM) parts and as replacement parts. Driving the accessories at very high speeds results in a substantial decrease in the efficiency of the vehicle because a substantial percentage of the output is required for the accessory drive, which can amount to up to about 30% of the engine horsepower at moderate speeds. This is extremely wasteful because the present fixed ratio drives results in accessory speeds which are not required for proper operation of the accessories.
This problem becomes even more severe in some modern engines which are of relatively low horsepower and which operate at relatively high speeds. Further, the lack of efficiency of the system requires the use of more gasoline--a needless and serious waste of energy. In addition, each accessory normally has a most efficient or optimum RPM range and with normal systems the accessory is not within this range due to wide variation in the input speed.
Various energy saving types of accessory drives have been proposed. For example, the drive between the engine and the fan for cooling the radiator has been thermostatically controlled. This type of drive is temperature dependent and has no relation to engine shaft speed. Such drive is not suitable for alternator or generator drive because these accessories must be driven continuously when the engine is operating. Other types of drives employ slipping friction clutches; while they may be successful, they have not found acceptance because of the cost of the drive and energy losses during operation.
For an accessory drive system to meet requirements for use in modern vehicles, it should be small enough to fit in present-day engine compartments without any substantial modifications, it should be relatively inexpensive, it should be susceptible of mass production and adjustable to modern assembly techniques, it should have long life, and it should produce a drive from the engine to the accessories which increases their speeds in approximately a linear relation with increasing engine speed at low RPM but which produces relatively constant accessory speeds as the engine speed increases above a predetermined point throughout the normal vehicle driving range. The present invention meets all these criteria.
The drive of this invention is to be distinguished from the conventional variable pulley transmission as is presently used in, for example, off-the-road vehicles or has been proposed for the transmission for transmitting power from an engine to the driving mechanism of a vehicle, whether they are wheels, lugs or other devices. In such prior transmissions, the engine rpm is increased and, at the same time, the rpm of the driving mechanism is increased at an even faster rate. Further, until a certain driveR pulley rpm is acheived, no power is transmitted to the driveN pulley.